I never seem to have them last more then 4 years. Usually without notice they start to die, I have gotten so use to it I now just toss them after 4 years. The only exception is maybe with my Optima's, they are within a month of their fourth year, I will have to let you know one that one. I would always replace them as a set two, it is just a matter of time for the other one, and why take the chance that it could affect the new one, just go ahead and replace the two as a set. Also, on the Dodge's the alternator is controlled by the PCM, this is a very expensive part to replace. I would not let a battery jeopardize your trucks electrical system. When batteries go bad the basically short from one cell to another, this can effect the computers and PCM, shorts cause voltage fluctuations and spikes, neither of which are good for the electrical components.
Yes they are expensive, but you have to have them. Go on and do the job right, replace them both with the highest quality batteries you can afford. So far the optima's are going a great job, you may want to consider them.